The invention concerns a method and an apparatus for decorating objects with a print image produced by screen printing.
The length of the print image in the peripheral direction of the object also determines the length of the stencil representing that print image in the printing screen. The object is rolled against that printing screen during the printing procedure. That gives rise to a certain minimum length for the printing screen. It will be appreciated, however, that this is greater than the length of the print image because, at each end thereof, the printing screen has an additional portion which is generally impermeable to printing ink. The additional portion which is at the front in the transport direction of the objects receives the ink which is intended for the respective printing operation and which is distributed over the print image in the printing screen during the printing operation by the squeegee. The squeegee is applied to the printing screen in that portion, and the ink is brought into contact with the object through the open meshes representing the print image. The additional portion of the printing screen, which is downstream of the print image in the transport direction, serves to receive the excess ink which is displaced by the squeegee into that portion. The lifting movement of the squeegee from the screen, which occurs after the conclusion of the printing operation, can also be performed in that portion. The ink which has not been used at the end of the printing operation can be removed in the usual way from that portion, for example by a discharge conduit, through which the excess ink flows from the printing screen into a storage container from which it can be returned to the printing screen. The man skilled in the art is familiar with the means and measures required for that purpose.
Known screen printing machines are so designed that all-round printing can be effected in a printing operation. The expression “all-round printing” means here that, in the printing operation, in the printing station, the object is rotated through 360° about its longitudinal axis. In that case, there is the possibility of the object being provided with printing over its entire periphery in one printing operation. In this case then, the stencil in the printing screen is also of a length corresponding to the length of the total periphery. The resulting print image, however, can also extend only over a part of the total periphery, insofar as it is shorter and/or has interruptions. For example, in such a way, the object is provided with at least two print image portions at a spacing from each other in the peripheral direction. The configuration of the final print image is dependent on the respective factors involved and can be implemented, for example, for decorative reasons or also for reasons relating to printing technology. Such reasons apply for example when, as is usual in particular in relation to drinks bottles, the objects have irregularities, caused by the manufacturing process, on the surface to be decorated. Those irregularities can be caused, for example, by the mold in which the bottles are produced.
That known possible way of all-round printing on the object presupposes that there is a minimum spacing between the objects which are to be successively introduced into the printing station. The minimum spacing takes account of the length of the overall printing screen, including the holder carrying it, for example in the form of a frame, because the respective following object can be brought into contact with the stencil, at the beginning of the actual printing operation, only when the printing operation on the respectively preceding object is concluded.
It is usual for the decoration of objects, for example bottles, to be effected in directly connected relationship with bottle production. For that purpose, the apparatuses required for the individual production and treatment operations are connected in direct succession and are connected together to afford a production line by one or more transport means for the objects. Thus, the speed at which the objects are fed to the screen printing station is predetermined by the speed at which the objects leave the manufacturing apparatus, that is by the number of bottles produced per unit of time. In the apparatuses which are usual nowadays, particularly for producing glass bottles, that number is so high, for example 250 bottles/minute of a diameter of preferably 62-75 mm, that the spacings between two successive objects are so short that printing would not be possible with a screen printing stencil which is of dimensions for all-round printing, because the length of the printing screen or the stencil in the transport direction would be too great for the spacing between two successive objects.